The advent of "smart" circuit interrupters, such as described within U.S. Pat. No. 4,672,501 permits several such circuit interrupters arranged within a common enclosure to communicate with a central processor unit over a power line communication network or a twisted pair of conductors. The central processor is in the form of a field programming unit which continuously interrogates each of the interconnected circuit interrupters to determine the status of the individual circuit interrupter setpoint parameters as well as to determine whether a circuit interrupter has responded to interrupt circuit current to an associated piece of electrical equipment. An operator could then readily determine from the field programming unit which of the circuit interrupters has responded and the nature and magnitude of the overcurrent condition causing the circuit interrupter to respond.
In certain industrial environments it is customary to assemble a main circuit interrupter along with several branch circuit interrupters within a common enclosure such as within low voltage switchgear applications. The field programming unit is arranged within the enclosure and is interconnected with the main and branch circuit interrupters by means of a communications link to allow the condition and status of each of the circuit interrupters and the associated protected electrical equipment to be continuously monitored. The branch circuit interrupters communicate with the field programming unit and the main circuit interrupter for a variety of functions including Zone Selective Interlock. With Zone Selective Interlock function such as described, for example, in U.S. Pat. No. 4,468,714, an upstream circuit interrupter is delayed from operating to interrupt circuit current to allow a downstream circuit interrupter closer to the fault to interrupt the circuit. The trip-time characteristics of the individual branch circuit interrupters can be independently adjusted to insure that the circuit interrupter closest to the fault will interrupt before any of the upstream circuit interrupters. The use of such a communications link between the main and branch circuit interrupters within a common enclosure is described within U.S. patent application Ser. No. 674,475, filed Mar. 25, 1991, entitled "An Electronic Circuit Interrupter With Attached Terminal Connector Block". A good description of a transceiver for communicating between the circuit interrupters and the field programming unit is found within U.S. Pat. No. 4,583,232.
When the distributed branch circuit interrupters are adjusted for Zone Selective Interlock function, it is essential that the setpoints within the individual circuit interrupters are not changed. In many cases, the unauthorized changes to the branch circuit interrupters are only discovered after an interrupter has tripped unnecessarily or has failed to trip within the designed setpoint time or current parameters.
Accordingly, one purpose of the invention is to determine whether unauthorized changes to stored setpoint data within interconnected electronic circuit interrupters and a field programming unit have occurred and provide immediate indication of such changes to the field programming unit.